Every week I played against people who has advantage because he took steps during service motion. I believed he had advantage by doing that extra movements but not the others.
Today I saw a post on IG of a player moving when serving clearly jumping sideways and told that person who reposted (a tennis league admin) and had an argument over it. I said that the ITF rule stated that the person need to be at rest and then can only have slight movement. He disagree with me. Before I raised about the movement he said he knew tennis and foot fault is only when crossing the line. I guess many people does not realize about this rule.
This foot fault argument toward players can be pretty heated, stars got angry being called FF.
So searched about it and the most does not explain about the "at rest" and "walking" however the USTA clearly stated as below in 2017 interpretations:
https://www.usta.com/content/dam/usta/pdfs/20170502_USTAOfficiatingScenariosand%20Interpretations_2017.pdf